Trouble populating 2D array for Match-3 Game

Hi All,

I'm trying to teach myself the mechanics behind a match-3 game and am having some issues handling my arrays. The code below seems to create my 2D array (at least, it gets through the CreateBoard function...) but when I go to try to populate it, things fall apart:

var numSymbols:int = 4; var symbols:Array = new Array(numSymbols); var currentBoard:Array;

 //the symbols being used atm.
 symbols[0] = "triangle";
 symbols[1] = "circle";
 symbols[2] = "square";
 symbols[3] = "ex";

 public function CreateBoard(x:int, y:int):Array {
     //create an array with of x elements. 
         //Each element will be an array of y elements.
     var gameBoard = new Array (x);
     for (var i in gameBoard)
         gameBoard = new Array(y);
     print("created");
     return gameBoard;
 }

 // 'a' is never getting a proper variable passed to it, it seems. 
 public function PopulateBoard(a:Array):Array{
     var toPop:Array = a;
     for (var i in toPop){
         for (var j in toPop[i])
             toPop[i] = symbols[Random.Range(0,numSymbols)];
     }
     print("populated");
     return toPop;
 }

 //I think I broke it. Just fix this, then start testing the output.
 function Update(){
     if (Input.GetButton("Fire1")){
         currentBoard = CreateBoard(5,4);
         currentBoard = PopulateBoard(currentBoard);
     //for (var i in currentBoard) print(currentBoard);
     }
 }

The specific error I'm getting is

NullReferenceException: Object reference not set to an instance of an object
Boo.Lang.Runtime.RuntimeServices.CheckNumericPromotion (IConvertible convertible)
Boo.Lang.Runtime.RuntimeServices.CheckNumericPromotion (System.Object value)
Boo.Lang.Runtime.RuntimeServices.UnboxInt32 (System.Object value)
gameLogic.PopulateBoard (UnityScript.Lang.Array a) (at Assets/gameLogic.js:24)
gameLogic.Update () (at Assets/gameLogic.js:35)

To me it sounds like the 'a' in PopulateBoard isn't getting passed the proper information but, as far as I can tell, it should be...

I decided to use JS arrays as, from what I understand, unity's built-in ones don't have as much flexibility. Also, I've seen the question here: http://answers.unity3d.com/questions/16431/passing-a-multidimensional-array-in-javascript but I'm trying to understand why my approach wouldn't work :(

Can someone show me the error of my ways?

Thanks in advance and sorry for the massive wall-o-code